Using multiple small LiPos

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dcr raptor
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Location: Capel le Ferne

Using multiple small LiPos

Postby dcr raptor » Wed Oct 11, 2017 1:16 pm

Hello,

Since the design of one of my robots is too small for the batteries I currently use, I've needed to look for smaller alternatives.

I've managed to find some 3.7V 100mAh LiPos that should work, however to get the power I need, I will need to use at least 2 LiPos.

I've come up with the following solutions:
2 in series - 7.4V 100mAh
2 in parallel - 3.7V 200mAh
4 LiPos with 2 series of 2 batteries wired in Parallel - 7.4V 200mAh
Circuit 1.png
Circuit 1.png (3.42 KiB) Viewed 213 times


Please can you give me some guidance as to what would be safe and suitable for a typical 4 wheel push-bot or a 2 wheeled non-spinner?

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BeligerAnt
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Re: Using multiple small LiPos

Postby BeligerAnt » Wed Oct 11, 2017 9:11 pm

Generally best not to connect cells in parallel. They will have (slightly) different voltages and the one at the higher voltage will discharge into the one at the lower voltage - with nothing to limit the current!
No problem with connecting cells in series - that's what all battery packs do.
7.2V @ 100mAh is enough for a 2WD robot, although you may have to replace/recharge batteries between fights.
I've never built a 4WD robot so can't comment on needed capacity. I'm sure someone else will chime in...

One thing to look for is the discharge rate of the cells. This is specified in "C" and is a multiplier of the capacity expressed as current. So, for example, a 100mAh battery with a 10C discharge rate can operate at 100 x 10 = 1000mA (peak) although you won't get that much current for very long! For modern cells with discharge rates of 25-30C this is less of a problem, but it's something to be aware of.
Gary, Team BeligerAnt

dcr raptor
Posts: 50
Joined: Fri Jan 04, 2013 3:21 am
Location: Capel le Ferne

Re: Using multiple small LiPos

Postby dcr raptor » Thu Oct 12, 2017 11:51 am

Discharge rate appears to be 2C, so it will peak at 200mA.

I'm planning to use Pololu HP motors.
Motor stats are as follows:

Free-run speed @ 6V:
1000 rpm

Free-run current @ 6V:
120 mA

Stall current @ 6V:
1600 mA

Stall torque @ 6V:
9 oz·in

Motor type:
1.6A stall @ 6V (HP 6V)

Would a battery of 7.4V, 100mAh, 2C be able to run these for the duration of a fight?

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BeligerAnt
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Re: Using multiple small LiPos

Postby BeligerAnt » Thu Oct 12, 2017 7:25 pm

Two motors at 120mA each (free-running, so about the lowest current you are likely to see) is more than 200mA. Are you sure about the 2C rating, as it's very low. Can you post a link to the battery specs?
If the discharge rate (different to the charge rate) really is only 2C then the batteries will not be suitable for any competitive antweight robot.
Gary, Team BeligerAnt

dcr raptor
Posts: 50
Joined: Fri Jan 04, 2013 3:21 am
Location: Capel le Ferne

Re: Using multiple small LiPos

Postby dcr raptor » Thu Oct 12, 2017 10:54 pm

The batteries I'm looking at are found here:
https://www.powerstream.com/thin-lithium-ion.htm

I'm looking at the 100mAh batteries, though since it doesn't look like these will be decent alternative for an ant, I'll probably need to redesign the body.

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BeligerAnt
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Re: Using multiple small LiPos

Postby BeligerAnt » Fri Oct 13, 2017 8:26 pm

It is the very thin size of those batteries that limits the discharge rate. They will (just) cope with 10C discharge so they might just work in an antweight as long as you change them between every fight.
Gary, Team BeligerAnt


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